Heretofore, there have been developed various means for obtaining high duty cast iron, i.e., spheroidal graphite cast iron, including a refining method using a slag containing oxides, cyanides, carbonates and fluorides of alkali metals and alkaline earth metals and/or lime nitrogen; a treatment method using MgF.sub.2 and/or CaSi.sub.2 ; a treatment method using AlF.sub.2, CdF.sub.2 and/or ZnF.sub.2 ; and a treatment method using lime silicide, lime fluoride and/or Mg-containing silicon iron. These conventional means, however, use fluorides to lower the melting point of the slag, which means that disposal of the slag in its non-treated condition cannot be effected without involving problems of environmental pollution. Technically, each means applies desulfurization and spheroidization treatments, but these treatments do not necessarily provide stabilized results depending upon the conditions involved. Further, they require troublesome operations.